Depends in what sense NT
Take a sheltered housing scheme. You have the scheme owner (the freeholder) responsible for the communal areas, and the resident (leasholder) who owns their own flat. So who is responsible for the maintenance and repair of the fire seperation between floors above the leaseholders flat ? (i.e; that big whacking great concrete slab).
I would suggest it is the freeholder, as it is a common area which serves more than just one leaseholder and is beyond the control of a leaseholder.
If youre talking totally seperate occupiers with a party wall between them, then clearly that wall is effectively the mercy of each occupiers, both could knock whacking great holes in it. But I cant see that as a common area, yes its common to both occupiers, but to me a common area exists in leaseholder / freeholder scenarios.
Not sure that really answers your question NT.